Department of Environmental Science, Faculty of Science, Institute for Water and Wetland Research, Radboud University Nijmegen, Nijmegen, The Netherlands.
PLoS One. 2013 May 28;8(5):e62413. doi: 10.1371/journal.pone.0062413. Print 2013.
One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.
海草床的一个最常被引用的生态系统服务功能是其对海岸的保护价值。许多研究强调了地上茎在衰减波浪、促进沉积和防止侵蚀方面的作用。这就提出了一个问题，即如果地上部分短而密度低（被放牧）、大部分生物量在地下组织中的海草床是否也能稳定沉积物。我们通过结合典型热带珊瑚礁滩的操纵性现场实验和波浪测量来研究这个问题，在那里绿海龟密集地吃草海草冠层。我们在垂直于海滩的横断面实验性地操纵波浪能量和放牧强度，并在离海滩三个距离的植被和实验创建的裸露地段之间比较沉积物床面的变化。我们的实验表明，i）即使是地上部分短而生物量低且受到严重放牧的海草植被也能将波浪引起的沉积物侵蚀减少高达三倍，ii）侵蚀是沿着植被丰富的珊瑚礁滩的位置的函数。在其他研究强调海草冠层对海岸线保护的重要性的地方，我们对开阔、低生物量且受到严重放牧的海草床的研究强烈表明，地下生物量对沉积物的固定也有很大的影响。这些结果表明，与浅无植被的近岸珊瑚礁滩相比，短而低生物量海草草地的存在保持了更高的床面水平，在到达海滩之前衰减波浪，从而降低海滩侵蚀率。我们建议重新考虑仅将地上生物量作为评估海岸保护服务价值的代理。
